At a quick glance, the Sweet Ambar Blue SDDM theme has two versions -- one for Plasma 5 and another one for Plasma 6. You probably want the one for Plasma 6. You can check which version of Plasma you're running in System Settings -> About this System -> KDE Plasma Version.

Be extremely careful when installing Plasma/SDDM themes. They are user-submitted, not always reviewed, and can contain arbitrary code. There have been incidents involving ~~malicious~~ damaging code downloaded through Plasma global themes.

I'm running Fedora 40 with kde plasma DE if it helps any

As the message is saying, you should try installing QtQuickControls

You are on Plasma 6 and seemingly all SDDM themes are currently broken.

Use the default theme and try to find ones that work.

Have you tried sudo dnf upgrade QtQuick.Controls Then Reboot If that doesn't work sudo dnf reinstall QtQuick.Controls Reboot again Still no worky Try moving the theme file out of the share folder and reassigning the directory to the new location Then reboot Post any error messages received in terminal

When I upgraded to Plasma 6, I needed to select default theme, upgrade, then re choose my theme. Also upgraded from command line rather than a terminal window.

Have you tried reinstalling sddm and/or the theme? Had the same kind of error a few weeks ago were reinstalling did the trick for me

Just a suggestion...if you've enabled disk encryption during installation, consider enabling autologin for SDDM so you're not having to enter two credentials at boot. You'll rarely ever see your greeter again, so it won't really matter which theme is used.

And if you did not enable disk encryption, consider doing so as the security and privacy benefits are significant for most users.

Shoo, it’s been a rough week for sddm over here…

I've had some VERY similar issues with SDDM ever since I switched to Fedora 40 Sway. Only my issue is with Qtgraphicaleffects, while quickcontrols seems to load properly. I have the following package: qt5-qtquickcontrols2 installed with dnf and it works. Hope it works for you too. But for me, I'm still having Qtgraphicaleffects issues, to the point I ended up rewriting my sddm theme without Qtgraphicaleffects. It's crazy, I know, but it's hard enough to find a decent theme without KDE dependencies as it is (I'm not using KDE Plasma). Finding one without qtgraphicaleffects as well would be even harder.